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Masters Theses Dissertations and Theses
July 2016
Micropaleontology and Isotope Stratigraphy of the Upper Aptian to Lower Cenomanian (~114-98 Ma) In ODP Site 763, Exmouth Plateau, NW Australia
Ali Alibrahim University of Massachusetts Amherst
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Recommended Citation Alibrahim, Ali, "Micropaleontology and Isotope Stratigraphy of the Upper Aptian to Lower Cenomanian (~114-98 Ma) In ODP Site 763, Exmouth Plateau, NW Australia" (2016). Masters Theses. 361. https://doi.org/10.7275/8392568 https://scholarworks.umass.edu/masters_theses_2/361
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MICROPALEONTOLOGY AND ISOTOPE STRATIGRAPHY OF THE UPPER APTIAN TO LOWER CENOMANIAN (~114-98 MA) IN ODP SITE 763, EXMOUTH PLATEAU, NW AUSTRALIA
A Thesis Presented
by
ALI H. ALIBRAHIM
Submitted to the Graduate School of the University of Massachusetts Amherst in partial fulfillment of the requirements for the degree of
MASTERS OF SCIENCE
May 2016
GEOSCIENCES
1
MICROPALEONTOLOGY AND ISOTOPE STRATIGRAPHY OF THE UPPER APTIAN TO LOWER CENOMANIAN (~114-98 MA) IN ODP SITE 763, EXMOUTH PLATEAU, NW AUSTRALIA
A Thesis Presented
by
ALI H. ALIBRAHIM
Approved as to style and content by:
______R. Mark Leckie, Chair
______Steven Petsch, Member
______Robert M. DeConto, Member
______Steven Petsch Graduate Program Director, Geosciences Department
2 DEDICATION
For my wife and daughter, for filling my life with love and happiness. For my mother and father who raised me up, taught me through my early years and encouraged me to learn everyday.
iii ACKNOWLEDGEMENTS
I thank Professor Mark Leckie for his kind leadership and guidance and for all the educational discussions and seminars he conducted. I also thank every member of the micropaleontology lab for helping me through all the challenges, Andy Fraass, Renata DeMello, Serena Dameron, Amanda Parker, Chris Lowery and Khalifa Elderbak. I appreciate the support of my committee members Professors Steven Petsch and Rob DeConto. My research benefited greatly from the Bio-geochmistry class of Professor Steven Petsch and the isotope geochemistry class of Professor Stephen Burns.
I thank Dr. Geraint Wyn ap Gwylim Hughes for introducing me to the field of micropaleontology in Saudi Aramco.
I thank the people at Saudi Aramco who approved my scholarship and trusted me to successfully conduct long term research in micropaleontology.
iv ABSTRACT
MICROPALEONTOLOGY AND ISOTOPE STRATIGRAPHY OF THE UPPER APTIAN TO LOWER CENOMANIAN (~114-98 MA) IN ODP SITE 763, EXMOUTH PLATEAU, NW AUSTRALIA
MAY, 2016
ALI H. ALIBRAHIM, B.Sc., UNIVERSITY OF LEEDS
M.S., UNIVERSITY OF MASSACHUSETTS AMHERST
Directed by: Professor R. Mark Leckie
The biostratigraphy and isotope stratigraphy of the upper Aptian to lower Cenomanian interval including oceanic anoxic events OAE1b, 1c and 1d are investigated in ODP Site
763, drilled on the Exmouth Plateau offshore northwest Australia. Benthic foraminifera suggest that Site 763 was situated in outer neritic to upper bathyal water depths (~150-
600 m). OAEs of the Atlantic basin and Tethys are typically associated with organic carbon-rich black shales and 13C excursions. However, OAEs at this high latitude site correlate with ocean acidificationδ and/or pyrite formation under anoxic conditions rather than black shales. Ocean acidification maybe responsible for sporadic low abundances of planktic foraminifera compared to radiolarians and benthic foraminifera associated with increased volcanogenic CO2 production during the formation of the
Southern and Central Kerguelen Plateaus. Sea surface temperature may have cooled to
11°C in the late Aptian but increased gradually during the Albian. The Aptian/Albian boundary is placed at a negative carbon isotope excursion associated with the lowest occurrence of Microhedbergella renilaevis, typically found within the Niveau Kilian black shale of OAE1b. Third-order sea level cycles, particularly in the middle Albian, produced cyclic changes in the abundance of inoceramid prisms that increased during inferred times of falling sea level. The late Albian OAE1c and OAE1d coincide with horizons of intense pyritization and the absence of all biocomponents suggesting the
v development of euxinia. Warm Tethyan waters reached the Exmouth Plateau during the latest Albian based on the presence of thermocline dwelling keeled planktic foraminifera including Planomalina buxtorfi.
vi TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ...... iv
ABSTRACT ...... v
LIST OF TABLES...... ix
LIST OF FIGURES ...... x
CHAPTER
1. INTRODUCTION ...... 1
1.2 The Aptian/Albian OAE1b ...... 6 1.3 Cooling in the late Aptian and warming across the Aptian/Albian boundary ...... 8 1.4 The early late Albian OAE1c ...... 10 1.5 The latest Albian OAE1d ...... 10 1.6 Study Site ...... 11 1.7 Research Questions ...... 14
2. MATERIALS AND METHODS ...... 15
2.1 The micropaleontology survey ...... 15 2.2 The isotope survey ...... 16 2.3 Paleotemperature Calculation ...... 17 2.4 Age Model ...... 17
3. RESULTS AND DISCUSSION...... 19
3.1 Age Model ...... 19 3.2 Micropaleontology ...... 20 3.3 Isotope Stratigraphy ...... 25 3.4 Isotopic composition ...... 25 3.5 Temperature Calculations ...... 26 3.6 Isotopic Gradients ...... 26 3.7 Chronostratigraphy ...... 30 3.7.1 OAE1b Kilian Event (Aptian/Albian boundary) ...... 30 3.7.2 The Albian Sub-stage and Albian/Cenomanian boundary ...... 33 3.8 Albian Oceanic Anoxic Events ...... 35 3.9 Paleoenvironment ...... 40 3.9.1 Micropaleontology ...... 40 3.9.2 Stable Isotope Record ...... 46 3.10 Comparison of Isotope Records of Bulk Carbonate to Bulk Fine Fraction Carbonates and Benthic Foraminifera ...... 53 3.10.1 Carbon Isotopes ...... 53
vii 3.10.2 Oxygen Isotopes ...... 54 3.11 Paleoenvironment across the Aptian/Albian OAE1b...... 59 3.12 Late Aptian Cooling ...... 62 3.13 Pyrite and Siderite as Expressions of Anoxia...... 62
4. CONCLUSIONS ...... 65
APPENDIX: AGE MODEL RESULTS ...... 68
REFERENCES ...... 71
viii LIST OF TABLES
Table Page
1. A list of globally established OAEs within the study interval...... 35
2. A list of the ages and depths of OAEs coinciding with severe drops in the total number of foraminifera ...... 35
ix LIST OF FIGURES
Figure Page
1. Mid- rocks and associated organic matter ...... 2 Cretaceous OAEs exhibit excursions in the δ13C record of carbonate 2. The source of organic matter in black shales is different based on sea level ... 3
3. Submarine volcanism drives hydrothermal activity, which leaches metal micronutrients seeding marine organic matter production leading to bottom water anoxia...... 4
4. In the Vocontian Basin, OAE1b consists of four black shales; the Jacob, Kilian, Paquier and Leenhardt...... 5
5. The percentage of planktic foraminifera and isotope stratigraphy across Niveau Kilian in the Col de Pré-Guittard section in the Vocontian basin...... 6
6. Water column stratification across OAE1b black shales in ODP Hole 1049C is indicated by the divergence between the isotopic signature of benthic and planktic foraminifera (deep and shallow dwellers) ...... 8
7. Paleotemperature estimates suggest a late Aptian cooling due to the O- isotope enrichment of planktic foraminifera and convergence with benthic values. 9
8. The collapse of near surface ocean stratification is illustrated in the overlap of deep and shallow dwelling planktic foraminifera across OAE1d black shales in ODP Site 1052 ...... 11
9. Bathymetric map of the Exmouth Plateau showing ODP Site 763 off the northwest coast of Australia ...... 12
10. Images of the interval surrounding the Aptian Albian boundary showing poor recovery of the uppermost Aptian ...... 13
11 Nannofossil events from three ODP sites from Exmouth Plateau including ODP 763B...... 18
12. Left: Sedimentation rates generally decrease up-section through the Albian and lower Cenomanian. Right: A depth-age plot of all samples in the study using nannofossil event datums published in Bralower and Seisser, 1992...... 19
13-1. Abundance data for all five major components vs depth (mbsf)...... 21
13-2. Percentage data plotted against deph for all five major components, in addition to keeled planktics...... 22
x 14-1. Benthic foraminifera counts are dominated by eight main genera, Osangularia, Gavelinella, Gaudryina, Gyroidinoides, Dorothia, Globorotalites, Stensoina and Lenticulina...... 23
14-2. Benthic taxa percentage data plotted against age ...... 24
15. Isotope records for the study interval...... 28
16. Temperature estimates from oxygen isotopes show high variability up to 530 mbsf. 29
17. Carbon and Oxygen isotopic gradients between mixed layer calcareous nannoplankton represented by bulk fine fraction and deep waters represented by benthic foraminifera...... 29
18. SEM images of planktic forams reveal severe acidification(?) in the uppermost Aptian sample compared to the basal Albian sample...... 31
19 . A comparison of the carbon isotope and planktic foraminifera records from the Vocontian basin support the placement of the Aptian/Albian boundary at 530 mbsf...... 32
20. The Aptian/Albian boundary coincides with a negative excursion in carbon isotopes, increased abundance of benthic and planktic foraminifera, radiolarians, inoceramid prisms, and a significant decrease in sedimentation rates...... 34
21. Severe drops in the total number of forams are observed in ODP 763B possibly linked to OAEs...... 36
22. All OAEs (except for Leenhardt), coincide with increased dominance of radiolarians and benthics over planktics...... 38
23. All OAEs may be associated with ocean acidification due to low foraminifera counts and P/B and P/R ratios...... 39
24. A paleobathymetic model of benthic genera in the Australian margin suggests that most benthic foraminifera formed in outer neritic to upper bathyal marine conditions...... 42
25. Inoceramids indicate a middle to outer neritic bathymetry...... 43
26. A gradual increase in infaunal benthics (Gaudryina and Dorothia) suggests lower oxygenation and increasing organic matter flux to the sea floor ...... 44
27. The abundance of inoceramid prisms is associated with a decline in planktic and radiolarian abundances suggesting that inoceramid prisms may reflect changes in sea levels...... 45
xi 28. A composite plot of isotope data and mineral and biocomponents with sequence stratigraphic interpretations...... 51
29. A composite plot of isotope, mineral and biocomponent data with acidification events...... 52
30. A composite record of data produced in this study compared with Clarke’s bulk carbonate isotope values...... 57
31. Clarke’s bulk isotope data are plotted with bulk fine fraction data from this study...... 58
32. The Aptian/Albian boundary Niveau Kilian black shale in the Vocontian Basin (lower middle plot) and equivalent horizons in the Mazagan Plateau (lower right) and the Newfoundland Basin (left) show negative excursions due to the input of isotopically light carbon from a combination of possible sources, including volcanogenic CO2 associated with Kerguelen Plateau volcanism, liberation of methane hydrates due to rising bottom water temperatures, and/or erosion of terrestrial organic due to falling sea level...... 61
35. Hydrothermal activity associated with the Southern Kerguelen Plateau could theoretically have supplied large amounts of Fe(II) in the late Aptian, which oxidized in shallower environments forming Fe(III), followed by hydrolysis forming Fe(OH)3 that sunk to the ocean floor...... 64
xii CHAPTER 1
INTRODUCTION
Oceanic Anoxic Events (OAEs) are intervals of widespread burial and preservation of
organic carbon under anoxic conditions typically forming black shales with ~1.0-2.5‰ excursions (positive or negative) 13C record of carbonates and associated
organic matter (Figure 1) [e.g., Bralowerin the δ et al., 1993; Leckie et al., 2002; Erba, 2004;
Jenkyns, 2010; Huber and Leckie, 2011] . Schlanger and Jenkyns (1976) introduced the term “Oceanic Anoxic Events” to describe two intervals of organic rich black shale deposition around the world that clustered around the Aptian/Albian (OAE1) and the
Cenomanian/Turonian (OAE2) boundaries [Arthur and Schlanger, 1979] . Arthur and others (1990) subdivided OAE1 into three sub-events deposited as discrete black shales in the early Aptian, latest Aptian and late Albian and termed them OAE1a, 1b and 1c consecutively. OAE1c was reassigned as OAE1d, and OAE1c was assigned to a black shale interval in the early late Albian [Bralower et al., 1993; Erbacher and Thurow, 1997]. This thesis focuses on OAE1b, 1c and 1d in terms of micropaleontology and stable isotope geochemistry, and their implications for sequence stratigraphy, biostratigraphy, paleoclimatology, and paleoceanography.
Oceanic Anoxic Events are often deposited as black shales in pelagic or hemipelagic facies but their genesis, isotope content, and organic matter varies due to local and regional paleoceanographic and sedimentological conditions [Alexandre et al., 2011] .
Erbacher et al., 1996 divided OAEs in two categories: Productivity OAEs (P-OAEs) and
Detrital OAEs (D-OAEs) (Figure 2). P-OAE black shales typically contain type II kerogen of marine origin, have positive 13C excursions in carbonates and organic matter, and
are typically deposited in transgressiveδ systems tracts. D-OAE black shales typically
contain type III kerogen of terrigenous origin, have negative 13C excursions in
δ 1 carbonates and organic matter, and are deposited in lowstand systems tracts (Figure 2).
Both OAE1b and OAE1d are Productivity OAEs with a wider global distribution, while
OAE1c is a Detrital OAE with limited distribution [Erbacher et al., 1996; Bralower et al.,,
1993; Leckie et al., 2002].
Figure 1. Mid-Cretaceous OAEs exhibit excursions in the δ13C record of carbonate rocks and associated organic matter. The Strontium isotopic ratio (87Sr/86Sr) exhibits a negative shift due to higher rates of submarine volcanism, which coincide with the deposition of Aptian and Cenomanian/Turonian black shales [Leckie et al., 2002]
The temporal and spatial relationship between OAEs and mid-Cretaceous submarine volcanism was first noted by Schlanger and Jenkyns in 1976. Most mid-Cretaceous OAEs are clustered between 120-93 Ma, which coincides with higher seafloor spreading rates and magma flux during the Cretaceous Normal Superchron (120-80 Ma) [Larson, 1991a, b; Larson and Erba, 1999; Coffin et al., 2006] . Large Igneous Provinces (LIPs) had the largest impact on Cretaceous environments due to their massive basaltic eruptions that
2 released greenhouse volatiles such as CO2, CH4 and SO2 leading to global warming
[Coffin et al., 2006] . In addition, higher hydrothermal activity increased the supply of micronutrients, including dissolved iron, which enhanced organic matter production and expanded the oxygen minimum zones leading to oceanic anoxia and deposition of organic rich horizons (Figure 3) [ Leckie et al., 2002; Bralower, 2008] . The strontium isotope ratios (87Sr/86Sr) of planktic, benthic and inoceramid bivalve shells indicate higher rates of submarine volcanism coinciding with the deposition of Aptian and
Cenomanian-Turonian OAEs, followed by higher rates of continental weathering during the Albian associated with rising sea level and global warming (Figure 4)[Bralower et al., 1997; Jones and Jenkyns, 2001; Leckie et al., 2002] .
Figure 2. The source of organic matter in black shales is different based on sealevel. A: A transgression leaches nutrients from lowlands to seed marine organic matter production leading to anoxia. B. Terrestrial organic matter is transported and redeposited on the shelf edge during a low stand systems tract [Erbacher et al., 1996]
3
Figure 3. Submarine volcanism drives hydrothermal activity, which leaches metal micronutrients seeding marine organic matter production leading to bottom water anoxia. CO2 is released to the atmosphere, which leads to global warming and intensification of water column stratification, which assists black shale deposition [Bralower, 2008]
4
Figure 4. In the Vocontian Basin, OAE1b consists of four black shales: Jacob, Kilian, Paquier and Leenhardt. The Kilian is associated with the largest of planktic turnover in the Aptian/Albian boundary interval [Leckie et al., 2002; Huber and Leckie, 2011; Petrizzo et al., 2012; Kennedy et al., 2014]
5 1.2 The Aptian/Albian OAE1b
The Aptian/Albian boundary interval (113.3 – 109.0 Ma) consists of four black shale
horizons that collectively define OAE1b (sensu Leckie et al., 2002) in the Col de Pre-
Guittard section in the Vocontian Basin, SE France [Bréhéret, 1994; Kennedy et al., 2000;
Herrle et al., 2004; Trabucho Alexandre et al., 2011; Petrizzo et al., 2012; Coccioni et al.,
2014; Kennedy et al., 2014] . The black shale horizons are deposited as follows: Niveau
Jacob (uppermost Aptian), Niveau Kilian (Aptian/Albian boundary), Niveau Paquier
(lowermost Albian) and Niveau Leenhardt (upper lower Albian) (Figure 5) [Petrizzo et al., 2012; Kennedy et al., 2014]. Each black shale horizon is enriched in organic matter deposited under anoxic conditions and correlates with negative excursion in the 13C
record separated by brief periods of oxygenation and pelagic carbonate depositionδ
[Moullade et al. 2011; Coccioni et al., 2014; Kennedy et al., 2014] .
Figure 5. The percentage of planktic foraminifera and isotope stratigraphy across Niveau Kilian in the Col de Pré-Guittard section in the Vocontian basin. The range chart illustrates a planktic turnover across Niveau Kilian. Species illustrated: 1. Paraticinella eubejaouaensis (= P. rohri), 2. Pseudoguembelitria blakenosensis, 3. Hedbergella infracretacea, 4. Hedbergella aptiana, 5. Microhedbergella miniglobularis, 6. Mi. renilaevis [Petrizzo et al., 2012]
6 Niveau Kilian black shale coincides with the most significant global evolutionary
turnover (extinction plus speciation) of planktic foraminifera in the Cretaceous [Leckie
et al., 2002; Petrizzo et al., 2012; Huber and Leckie, 2011] . In addition, planktic foraminifera show a severe reduction in shell size, diversity and morphological complexity. The turnover of planktic foraminifera across Niveau Kilian can be correlated globally and is not limited to sites with black shale development [Petrizzo et al., 2012; Huber and Leckie, 2011] . The turnover consists of the following events: the
Highest Occurrence (HO) of Paraticinella eubejaouaensis (= P. rohri), Pseudoguembelitria blakenosensis, Hedbergella infracretacea, H. aptiana, and the Lowest Occurrence (LO) of
Microhedbergella miniglobularis and Mi. renilaevis (Figure 6) [Huber and Leckie, 2011;
Petrizzo et al., 2012] . In addition, the AABI can also be approximated by the LO of the circular form of the nannofossil Prediscosphaera columnata and the LO of the benthic foraminifera Pleurostomella subnodosa [Moullade, 1966; Kennedy et al., 2000; Moullade
2011; Huber and Leckie, 2011; Kennedy et al., 2014]] .
OAE1b black shales, in general, correlate with a global sea-level rise, rising
temperatures, increased stratification, and higher productivity. Stratification is well
established in the convergence of values of 13C and divergence of 18O records of
epifaunal and infaunal benthics versus deepδ and shallow dwelling plankticδ foraminifera
in one site from the western tropical North Atlantic ([Erbacher et al., 2001; Petrizzo et
al., 2012] .
7
Figure 6. Water column stratification across OAE1b Paquier black shale in ODP Hole 1049C is indicated by the divergence between the δ18O signature of benthic and planktic foraminifera (deep and shallow dwellers) [Erbacher et al., 2001]
1.3 Cooling in the late Aptian and warming across the Aptian/Albian boundary
The Cretaceous, in general, is typically viewed as a greenhouse world caused by increased ocean crust production, including LIP emplacement, and CO2 outgassing leading to rising temperatures and sea level (e.g., Larson, 1991a, b). However, there is a plethora of evidence for cool interludes in the latest Aptian with warming across the
8 Aptian/Albian boundary interval and through the Albian (Weissert and Lini, 1991;
Clarke and Jenkyns, 1999; Huber et al., 2002; Leckie et al., 2002; Price, 2003; Mutterlose et al., 2009; Friedrich et al., 2012; Petrizzo et al., 2012; McAnena et al., 2013) (Figure 7).
For example, 18O data of belemnites from shelf environments suggest paleotemperaturesδ between 7.7° and 11.3°C for the lower Albian Gearle Siltstone outcrops in northwest Australia (paleolatitude ~55°S) and the Rio Mayer Formation in southern Argentina (paleolatitude 57°S) [Pirrie et al., 1995; Pirrie et al., 2004] , while
Tethyan warm water nannoconids experienced a significant decline in latest Aptian
(Leckie et al., 2002) and cool water, high latitude taxa increased in abundance in the
Exmouth Plateau and the Vocontian basin [Mutterlose et al., 2009] .
Figure 7. Paleotemperature estimates suggest a late Aptian cooling due to the O-isotope enrichment of planktic foraminifera and convergence with benthic values. The sea surface temperature is ~17°C and is much cooler than typical tropical Cretaceous SSTs [Leckie et al., 2002] 9 A late Aptian cooling probably resulted from decreasing levels of CO2 by increased
organic matter burial and/or silicate weathering [Lini and Weissert 1991; Hong and Lee,
2012; Maurer et al., 2012; McAnena et al., 2013.]
1.4 The early late Albian OAE1c
OAE1c is an early late Albian (107.18 – 106.78 Ma) black shale horizon rich in detrital
organic matter, but unlike OAE1b and 1d, OAE1c is not associated with a turnover in
calcareous plankton or radiolaria [Bralower et al., 1993; Erbacher, 1996; Leckie et al.,
2002]. OAE1c is a DOAE that coincides with a sea level fall and higher sedimentation
rates in basins close to continental hinterlands (e.g., Mazagan Plateau, NW African continental margin) [Erbacher et al., 1996]. OAE1c is poorly known. The Toolebuc
Formation of the Queensland Basin, Australia, is an example of a regionally developed,
correlative oil shale interval [Haig, 1979; Bralower et al., 1993].
1.5 The latest Albian OAE1d
The latest Albian interval (103.3 – 99.0 Ma) consists of several black shale horizons
and a positive 13C excursion that define OAE1d (Figure 1) [Wilson and Norris, 2001;
Watkins et al., 2005;δ Petrizzo et al., 2008; Melinte-Dobrinescu, 2015] . OAE1d was first
described as a radiolarian turnover event across the uppermost Albian black shale
horizon in the Le Brecce section in the Umbria-Marche basin, Italy [Erbacher and
Thurow, 1997] . However, OAE1d is also associated with a major phase of biotic
turnover affecting planktic foraminifera including the HO of Biticinella and Ticinella and
the LO of Planomalina, Praeglobotruncana and Paracostellagerina, as well as turnover in
the Rotalipora [ Nederbragt et al., 2001; Leckie et al., 2002; Petrizzo et al., 2008] .
Unlike the Aptian/Albian boundary interval of multiple black shales of OAE1b, the
10 latest Albian black shales of OAE1d are not deposited synchronously and their 13C excursion peaks do not correlate with the deposition of black shales [Petrizzo etδ al.,
2008] . The convergence of 18O values of deep and shallow dwelling planktic and benthic foraminifera at one δNorth Atlantic site suggests the collapse of stratification and greater mixing in the upper water column and higher productivity [Wilson and Norris,
2001; Petrizzo et al., 2008] .
Figure 8. The collapse of near surface ocean stratification is illustrated in the overlap of deep and shallow dwelling planktic foraminifera across OAE1d black shales in ODP Site 1052 [Wilson and Norris, 2001]
1.6 Study Site
The Exmouth Plateau is a deep-water marginal plateau that is the westernmost extent of the North Carnarvon Basin, NW Australia. ODP Site 763 was drilled in 1988 during
Ocean Drilling Program Leg 122 (20°35.19'S, 112°12.52'E) P onlateau the Exm outh under 1367 m water depth to provide sedimentological and sequence stratigraphic data for the Cretaceous section (Figure 9) [Suess et al., 1988] . Three holes, A, B and C were drilled to a total depth of 1376.6 m with 121 cores representing the interval from the middle Berriasian to upper Eocene. The interval of interest for this research was only cored in Hole 763B between Cores 25X-41X (408.5-570.0 m) and spans the upper
11 Aptian-lower Cenomanian. Average core recovery was 81% for Hole 763B, but dropped
to an average of 26% for four consecutive cores in the upper Aptian (Cores 38X-
41X)(Figure 10) [Suess et al., 1988] .
The sedimentary succession in Hole 763B was divided into 7 units based on visual descriptions and smear slide sedimentological estimates. Unit 4 was assigned to the interval from (385.72-570.00 mbsf), which was subdivided into subunits 4A (385.72-
532.00 mbsf: Cores 23X-37X) and 4B (532.00-570.00 mbsf: Cores 38X-41X) based on the presence of hard carbonates and poor recovery in subunit 4B. No black shales were recovered, but the Aptian/Albian and the Albian/Cenomanian boundaries were tentatively picked shipboard at 570 mbsf and 420 mbsf, respectively, based on calcareous nannofossil and planktic foraminiferal data [Suess et al., 1988] .
The Aptian/Albian boundary at Hole 763B was later picked at 528 mbsf based on the
LO of Prediscosphaera columnata [Bralower and Siesser, 1992] , and supported by a planktic turnover at 530 mbsf [Huber and Leckie, 2011] .
Fi
Figure 9. Bathymetric map of the Exmouth Plateau showing ODP Site 763 off the northwest coast of Australia [Suess et al., 1988]
12
Figure 10. Top: Images of the interval surrounding the Aptian Albian boundary showing poor recovery of the uppermost Aptian. The Aptian/Albian boundary is placed in Section 763B-37X-5. Note the poor recovery in uppermost Aptian Cores 38X-41X.
Left: A lithostratigraphy column with the cored interval of interest. Core recovery is poor from the middle of Core 37X to the bottom of 42X as illustrated in the black and white column next to core numbers [Suess et al., 1988] . The Aptian/Albian boundary is placed in Section 763B- 37-5 (Huber and Leckie, 2011).
13 1.7 Research Questions
1. Do the assemblage and abundance data of benthic and planktic foraminifera
exhibit changes or turnovers at the intervals correlative to OAE1b, 1c and 1d?
2. What paleoceanographic and sequence stratigraphic interpretations can be
drawn from sedimentological, biogenic, and foraminiferal assemblages and
isotope data in ODP Hole 763B?
3. Do benthic and planktic foraminiferal assemblages and isotope records suggest
reworking and an unconformity across the Aptian/Albian boundary interval?
4. Is there evidence for latest Aptian cooling at ODP Site 763?
14 CHAPTER 2
MATERIALS AND METHODS
A total of 138 Samples from ODP Hole 763B Cores 25X-41X were analyzed in this
study with a maximum sample spacing of 1.58 m and an overall average of 1.22 m
producing a 160-kyr resolution in the Albian and Cenomanian intervals and 30-kyr resolution in the Aptian. 68 Samples were originally sampled and studied by Richard
Cashman and R. Mark Leckie on a 63 micron sieve [Leckie et al., 2002; Huber and Leckie,
2011]. 70 new samples were requested from the International Ocean Discovery
Program core repository in Kochi, Japan; each sample was labeled using standard IODP hole, core-section, interval conventions, as well as depth in meters below seafloor
(mbsf).
Samples were mechanically crushed, soaked for a week in 300 ml of Sparkleen solution, agitated daily to insure disintegration of clays in a basic solution, and then washed over a 63 µm sieve. Samples were washed a second time over a 63 µm sieve, soaked in 10 ml of water, then dried for 24 hours.
2.1 The micropaleontology survey
The micropaleontology survey was conducted on all samples by further sieving the samples on a 125 µm sieve mainly to investigate the response of benthic foraminifera to possible dysoxic-anoxic conditions across OAEs in the study interval. The study also acquired detailed counts on planktic forams, radiolarians, inoceramid prisms, and pyrite nodules.
Samples were split to achieve a smaller representative quantity of the sample, which is then analyzed on a gridded picking tray. A minimum of 300 foraminifera specimens were picked and counted regardless of the associated number of other components
15 such as radiolarians and inoceramid prisms, which exceeded 1000 counts in some
samples.
The number of all components were scaled up by a factor of 2 to the power of the
number of splits. For example, three splits provide a 1/8 fraction of the sample, so the
number of specimens found in the tray are multiplied by 8 to approximate the
population in the entire 10-CC sample. The total number of components provides an
accurate percentage of each component per sample. The number of planktic and benthic
specimens can be expressed P/B ratio, which is the percentage of planktic and benthic
specimens relative to the total number of foraminifera.
2.2 The isotope survey
The isotopic composition of the interval was conducted using 203 samples focused on
four different components at different resolutions, two epifaunal benthic forams,
Osangularia schloenbachi and Gavelinella spp., fine fraction (<63 µm) bulk carbonates
(Bulk FF), and Microhedbergella spp. Osangularia schloenbachi (epifaunal benthic
foram) was analyzed in 90 samples, and Gavelinella spp. (another epifaunal benthic)
was analyzed in 44 samples, for an average of 1.22 and 3 m intervals, respectively. Bulk
FF were sampled every 3 m or less (68 samples), and Microhedbergella spp. at variable
spacing (27 samples).
Osangularia is the most abundant epifaunal benthic genus in the study section, which
makes it ideal for a detailed isotope study; however, it is absent in the Aptian interval.
The long-ranging genus Gavelinella was sampled in the entire section (Aptian-
Cenomanian) to provide benthic isotopic signals in the Aptian and provide a supporting benthic isotopic signal in the Albian-Cenomanian in addition to Osangularia.
Fine fraction bulk carbonates (<63 µm) were collected during the washing phase and
16 were run for isotopic investigation every 3 m in the Albian-Cenomanian and every 0.86
m, on average, in the Aptian. Planktic isotopes were few because they were only used to
provide control on Bulk FF isotope data and their values generally agree.
All samples were run at the University of Missouri Stable Isotope Facility using a Kiel
III automated carbonate device coupled a Thero Finnigan DeltaPlus IRMS. Results were normalized by the difference between average replicate analyses of standard carbonate material NBS-19 and nominal values of -2.2‰ and +1.95‰ for δ18O and δ13C consecutively.
Individual forams were hand-picked until a sample’s weight ranged between 40-80
µg; specimens were then placed in glass vials and loaded in the reaction chamber. Each vial is flushed with helium to remove air before adding phosphoric acid (H3PO4). Three
to four drops of acid are reacted with each sample at 80°C for an hour. Liberated CO2 is
analyzed under a helium flow connected to a Mass Spectrometer.
2.3 Paleotemperature Calculation
Paleotemperatures can be estimated using the equation of Bemis et al. (1998) assuming the δ18Oseawater is -1‰ [Schackleton and Kennett, 1975; Friedrich et al., 2012;
Price et al., 2012]. However, the presence of ice caps and biological fractionations associated with planktic foraminifera may cause errors in temperature estimates.
° = 16.0 4.14( ) + 0.13 ( ) 2 푇 퐶 − 훿푐푐푐푐푐푐푐푐푐 − 훿푠𝑠푠𝑠𝑠 훿푐푐푐푐푐푐푐푐푐 − 훿푠𝑠푠𝑠𝑠 2.4 Age Model
An age model is essential in calculating sedimentation rates and estimating durations of unconformities. The age model for this study was established on the Lowest
Occurrence events (LO) of calcareous nannofossils from ODP Hole 763B (Bralower and
17 Siesser, 1992). A table of the nannofossil datums is shown in Figure 21. Numerical ages of the nannofossil events are based on the 2012 Geological Time Scale [Gradstein et al.,
2012] . Sedimentation rates are determined by
Equation1: = ( )
∆퐷�퐷�ℎ 푆푆푆푆푆푆푆푆푆푆푆푆푆 푅𝑅� ∆ 퐴퐴� 푏𝑏푏𝑏� 𝑡� 𝑛𝑛�푛�푛푛�푛 𝑑�푑𝑑 Sample age will be determined as
1 = ( ) +
푆푆푆푆�푆 �푎� � ∙ 푆푆푆푆�푆 𝑑𝑑ℎ − 퐷𝐷𝐷 𝑑𝑑ℎ � 푌𝑌𝑌𝑌 퐷𝐷𝐷 퐴𝐴 𝑠� 푟𝑟�
Figure 11 Nannofossil events from three ODP sites from Exmouth Plateau including ODP 763B. These events were used to construct the age model for the studied interval (400 – 560 mbsf) [from Bralower and Siesser, 1992] .
18 CHAPTER 3
RESULTS AND DISCUSSION
3.1 Age Model:
Sedimentation rates decreased significantly from 33 m/myr in the upper Aptian to ~5.6
m/myr in the Cenomanian with the lowest sedimentation rates (~5 m/myr.) in the
middle to lower upper Albian interval (484.6 – 457.8 mbsf, 109.4 – 103.1 Ma).
Sedimentation rates increased to 12.5 m/myr in the uppermost Albian (455.7-424.29),
but fell again to 5.6 m/myr in the early Cenomanian. The highest sedimentation rates in the section (33.2 m/myr) were found in the uppermost Aptian (543 – 530 mbsf, 113.4 –
113 Ma). The accuracy of the age model is lower in Cores 40X-41X due to the absence of
a nannofossil datum below the base of Seribiscutum primitivum (113.4 Ma, 543.7 mbsf,
Core 39X-CC). Sedimentation rates may be lower due to the higher abundance of clay
content observed as a prominent color change in core 41X. The age model was used to
convert depths of all samples to age (Appendix Table 1)
19
Figure 12. Left: Sedimentation rates generally decrease up-section through the Albian and lower Cenomanian. Right: A depth-age plot of all samples in the study using nannofossil event datums published in Bralower and Seisser, 1992. Datum ages are calibrated on the 2012 Geologic Time Scale [Gradstein et al., 2012]
19 3.2 Micropaleontology:
All mineral and biocomponents were classified into the following groups: planktic and
benthic foraminifera, radiolarians, inoceramid prisms, and pyrite nodules. Planktic forams were further divided into keeled and non-keeled groups (Figures 13-1, 13-2).
Benthic forams were split into eight primary genera: Osangularia, Gavelinella,
Gyroidinoides, Gaudryina, Stensoina, Globorotalites, Lenticulina, and Dorothia (Figures
14-1, 14-2)
Different biocomponents dominate parts of the section starting with a sporadic dominance of planktic forams in the upper Aptian, 563.46 – 530.28 mbsf (113.99 – 113
Ma) followed by a marked abundance of benthic forams in the basal Albian at 528.73 mbsf (sample 37X-5, 112.94 Ma), which then decline significantly at 525.7 mbsf (112.7
Ma). Radiolarians dominate the lower Albian part of section between 519 – 503 mbsf
(112.27 - 111.14 Ma) followed by pulses of abundant inoceramid prisms in the middle
Albian between 489 – 437.02 mbsf (110.04 - 101.46 Ma). Pyrite and keeled planktic forams dominate the uppermost Albian part of the section between 446.7 – 425 mbsf
(102.2 – 100.5 Ma).
Planktic forams are generally more abundant than benthics but their counts reveal rhythmic peaks in abundance every ~10 meters in the section. Benthic forams peak abundance was found in the lowest Albian sample in Core 37X-5 at 528.73 mbsf,
(112.94 Ma), followed by a significant decline over the interval 528.73 – 492 mbsf.
Changes in benthic foraminiferal abundance appear to correlate with changes in sedimentation rates and abundance of inoceramid prisms. Benthic taxonomy shows there are at least 3 horizons where abundances decrease in all taxa (530, 492, and 440 mbsf) (Figures 14-1 and 14-2).
20 122880
13440
Figure 13-1. Abundance data for all five major components vs depth (mbsf). Benthic foraminifera dominate the interval (530 – 525 mbsf) followed by a dominance of radiolarians (519 – 505 mbsf), followed by inoceramid prisms (490 – 455 mbsf) followed by pyrite and keeled planktics (455 – 425 msf)
21
Figure 13-2. Percentage data plotted against deph for all five major components, in addition to keeled planktics. Benthic and planktic percentages are present in the entire section and show apparently cyclic changes. Radiolarian percentages are high in the early Albian and early Cenomanian wihle Inoceramids often dominate the interval between the middle and late Albian. Pyrite and keeled planktic percentages dominate the uppermost late Albian.
22
5760 3712 2048
Figure 14-1. Benthic foraminifera counts are dominated by eight main genera: Osangularia, Gavelinella, Gaudryina, Gyroidinoides, Dorothia, Globorotalites, Stensoina and Lenticulina. Most taxa appear at the Aptian/Albian boundary and show trends that lead to disappearance at 493 and 440 mbsf.
23 Figure 14-2. Benthic taxa percentage data plotted against age. Osangularia is the dominant benthic genera in the section but it is absent in the Aptian. On the other hand, Gavelinella, Gaudryina, Gyroidinoides, and Lenticulina are present in the Aptian and show increasing percentages up-section.
24 3.3 Isotope Stratigraphy
The isotopic composition of both epifaunal benthics, Osangularia schloenbachi and
Gavelinella spp., is remarkably similar in both Carbon and Oxygen isotopes. The long- term trend of δ13C isotopes of benthics and bulk fine-fraction (FF) gradually decrease
upwards with sharp negative excursions in the upper half of the section. Oxygen
isotopes of bulk FF gradually decrease upwards through the Albian, while benthic
values are generally steady except for several negative excursions that coincide with
carbon isotope excursions.
3.4 Isotopic composition
Carbon and Oxygen isotope values can be divided in four sections:
• 563 – 530 mbsf
Carbonate bulk fine fraction data was sampled at high-resolution, however, the isotopic
values show dramatic variability in the upper Aptian part of the section. Planktic
Microhedbergella and benthic Gavelinella were present in the upper part of this interval
and their isotopic compositions are remarkably similar.
• 530 – 482 mbsf
A negative δ13C excursion at 530 mbsf is typical of OAEs followed by an increased
gradient in both carbon and oxygen isotopes between bulk FF and benthic forams
suggesting a significant change in water column stratification and carbon cycling.
• 482 – 448 mbsf
Three synchronous high amplitude negative excursions occur in benthic and bulk FF
except in bulk FF δ18O where isotopic show positive excursions that coincide or lag benthic excursions. An opposite trend occurs between these high amplitude excursions
25 where isotopic values and gradients increase.
• 448 – 410 mbsf
The gradient between δ18O values increases dramatically with increasingly depleted
values of bulk FF while benthics are gradually more enriched. The gradient between
δ13C values also increases significantly, however, bulk FF values become more depleted
up to 429 mbsf followed by steep enrichment up to 410 mbsf.
3.5 Temperature Calculations
The δ18O of calcite is related to the δ18O of the seawater from which it precipitated by
a temperature and/or salinity factor. However, the effect of salinity on benthic δ18O
values is excluded in this site due to its 1000 m paleo-water depth during deposition, which reduces the effect of evaporation and precipitation. Temperature variations can be divided into the same four categories used to define isotopic values. Assuming
δ18Oseawater = -1‰, bulk FF temperatures increase gradually from ~13° to 24°C, while
benthic temperature estimates increase from ~11° to 15° in the upper part of the
section. Bottom water temperatures show positive excursions of up to 4°C in the middle
part of the section (Figure 16)
3.6 Isotopic Gradients
Isotopic gradients are the differences between isotope values of benthic foraminifera
and mixed layer calcareous plankton illustrated in bulk FF, assumed to be largely
composed of calcareous nannoplankton and juvenile planktic foraminifera. Gradients in
δ13C may explain local changes in primary productivity or changing surface or
intermediate water masses, while δ18O gradients represent changes in the thermocline
and water column stratification.
26 Plots of both Carbon and Oxygen isotope gradients show three nadirs at 482, 467 and
451 mbsf. The carbon isotopic gradient collapses at the Aptian/Albian boundary at 530 mbsf while oxygen isotope gradients collapse above and below this horizon at 524 and
533 mbsf consecutively (Figure 17).
27
Figure 15. Isotope records for the study interval. Left: Carbon isotopes. Osangularia and Gavelinella values track each other very closely with a small offset in values. Benthic & bulk FF δ13C values decrease gradually in the long term but several high amplitude excursions interrupt the record. δ18O values 28
Figure 16. Temperature estimates from oxygen isotopes show Figure 17. Carbon and Oxygen isotopic gradients between mixed layer high variability up to 530 mbsf. Mixed layer temperatures calcareous nannoplankton represented by bulk fine fraction and deep waters increase gradually upwards with severe fluctuations in represented by benthic foraminifera. Calcareous plankton and benthic temperature between (492 – 448 mbsf). Benthic temperature foraminifera show three sharp excursions from 480-450 mbsf (gray bars), which estimates show slower temperature increases upwards coincide with benthic warming events (Figure 16). At 530 mbsf the δ13C gradient interrupted by high amplitude warming events between (479 – collapses while the δ18O gradient collapses above and below this horizon at 524 451 mbsf). and 533 mbsf.
29 3.7 Chronostratigraphy
3.7.1 OAE1b Kilian Event (Aptian/Albian boundary)
The Aptian/Albian boundary is placed at a suspected unconformity between Sample
37X-5 (lowest Albian sample, 528.73 mbsf, 112.95 Ma) and 37X-6 (highest Aptian sample, 530.28 mbsf, 113.00 Ma). SEM images of planktic foraminifera show a significant change in taxonomy and surface ultrastructure between these two samples
(Figure 18). The lowest occurrence of Microhedbergella renialevis, a lowermost Albian small, smooth, microperforate planktic foram with four chambers in the final whorl occurs at 37X-5 and is preceded by poorly preserved planktic specimens in 37X-6. The
Highest Occurrence of Hedbergella infracretacea is in the uppermost Aptian. This finely perforate taxon is covered by pore mounds on the shell surface, a characteristic feature of late Aptian species of Hedbergella. Microhedbergella miniglobularis has a narrow range straddling the Aptian/Albian boundary (uppermost Aptian Mi. miniglobularis zone, 113.26 Ma, to basal Albian Mi. rischi zone, ~112.4 Ma; Huber and Leckie, 2011;
Petrizzo et al., 2012; Kennedy et al., 2014) and it occurs in both samples (37X-5 and
37X-6) further supporting the proximity of both samples to the Aptian/Albian boundary and suggesting that any break in the record was short.
A negative ~2.5‰ δ13C excursion occurs in benthic and bulk FF data at the highest
Aptian sample 37X-6, 530.28 mbsf. This excursion correlates well with a negative 1.8‰
excursion between the Mi. miniglobualris and Mi. renialevis zones in the proposed GSSP
locality in the Col de Pré-Guittard section in the Vocontian basin, SE France[Petrizzo et
al., 2012] (Figure 20). The abundance and preservation of planktic and benthic
foraminifera changes drastically between 37X-5 and 37X-6 suggesting a dramatic shift
in the paleoenvironment across the Aptian/Albian boundary.
30 Microhedbergella miniglobularis Microhedbergella rischi Microhedbergella renilaevis
Sample 37X-5 528 Albian Albian mbsf
Unconformity
Microhedbergella miniglobularis Hedbergella aptiana Hedbergella infracretacea
Sample
37X -6 530 mbsf Aptian Aptian
Figure 18. SEM images of planktic forams reveal severe dissolution, or acidification(?) in the uppermost Aptian sample compared to the basal Albian sample. Microhedbergella miniglobularis occurs in both samples, showing poor preservation in the Aptian . Hedbergella aptiana and Hedbergella infracretacea are upper Aptian marker taxa. Microhedbergella rischi and Mi. renilaevis are both exclusively lower Albian taxa.
31
Mi. renilaevis
Figure 19. A comparison of the carbon isotope and planktic foraminifera records from the Vocontian Basin support the placement of the Aptian/Albian boundary at 530 mbsf. A negative carbon isotope excursion is observed in benthic and bulk fine fraction carbonates in ODP Hole 763B which coincides with the lowest occurrence of a zonal marker Microhedbergella renilaevis
32 3.7.2 The Albian Sub-stage and Albian/Cenomanian boundary
The early/middle Albian boundary (110.7 Ma) is placed at 489 mbsf, the middle/late
Albian boundary (107.6 Ma) is placed at 476 mbsf, and the Albian/Cenomanian boundary (100.5 Ma) is placed at 425 mbsf based on the calcareous nannofossil-based age model for Site 763, and supported by planktic foraminiferal biostratigraphy.
Plotting chronostratigraphic boundaries on a combined plot of isotope values, sedimentation rates, and biocomponent and mineral graphs reveals significant changes across OAE1b. In addition, the middle to upper Albian show an inverse correlation between isotope values and the abundance of inoceramid prisms. Pyrite nodules are more abundant in in the uppermost Albian where isotope values stabilize and gradients become larger. The Albian/Cenomanian boundary coincides with a drop in planktic foraminifera and pyrite nodule abundances (Figure 20).
33 A Composite Graph of Stable Isotopes, Biocomponents and Sedimentation Rates
Figure 20. The Aptian/Albian boundary coincides with a negative excursion in carbon isotopes, increased abundance of benthic and planktic foraminifera, radiolarians, inoceramid prisms, and a significant decrease in sedimentation rates. The early Albian has the highest abundance of radiolarians while the middle and early late Albian coincide with sharp isotope excursions, slower sedimentation rates and increased abundance of benthic forams and inoceramid prisms. Pyrite nodule counts and the percentage of keeled planktics increase in the latest Albian coinciding with a severe decline in Inoceramid prisms, stabilized isotope values an increasing gradient.
34 3.8 Albian Oceanic Anoxic Events
The Niveau Kilian black shale represents the Aptian/Albian boundary in the
Vocontian Basin of southeast France and it has been placed at sample 37X-6, 530.25
mbsf as discussed in the Chronostratigraphy section above. A table of numerical ages of
Vocontian Basin black shale horizons based on (GTS 2012) are listed below.
Table 1. A list of globally established OAEs within the study interval. Numerical ages are based on the 2012 Geological Time Scale [Gradstein et al., 2012] Horizon OAE designation Age top Age bottom Niveau Breistroffer OAE 1d 100.98 101.03 Niveau Amadeus OAE 1c 106.78 107.18 Niveau Leenhardt OAE 1b 110.23 110.28 Niveau Paquier OAE 1b 111.15 111.27 Niveau Kilian OAE 1b 113.02 113.14 All OAE horizons correspond with reduced abundances of planktic and benthic
foraminifera, OAE ages were used to investigate sharp drops in foraminiferal
abundances in Hole 763B. Indeed, a drop in planktic and/or benthic forams are
observed within ~160 kyr in our age calibrated samples relative to established global
ages of OAEs. The depths of horizons equivalent to OAEs are listed below including the
calculated difference between Site 763 age model relative to globally established ages of
OAEs.
Table 2. A list of the ages and depths of OAEs coinciding with severe drops in the total number of foraminifera. The Site 763 age model correlates to the Albian OAEs within ~0.16 myr. OAE Site 763 Standard Age Site 763 Age Difference in Total Horizon designation Depth (GTS 2012) Model age (myr) Forams Niveau Breistroffer OAE 1d 427.73 101.01 100.72 0.28 0 Niveau Amadeus OAE 1c 475.09 106.98 107.26 -0.08 1784 Niveau Leenhardt OAE1b 493.73 110.26 110.47 -0.19 64 Niveau Paquier OAE1b 503.23 111.21 111.14 0.13 1456 Niveau Kilian OAE1b 530.25 113.08 113.00 0.14 3072
A plot of the total foraminifera counts (benthics and planktics) shows abrupt drops
coinciding with globally established OAE horizons (Figure 20).
35
Figure 21. Severe drops in the total number of forams are observed in ODP 763B possibly linked to OAEs. The placement of each OAE horizon was based on the calculated age model.
An additional observation is that, during OAEs, planktic abundances decline more
sharply than radiolarian and benthic abundances resulting in lower
planktic/radiolarian ratios (P/R) and planktic/benthic ratios (P/B) (Figure 22). This
suggests that conditions associated with OAEs had a stronger effect on planktics than
benthics and radiolarians, further suggesting that the water column over Site 763 on the
Exmouth Plateau was not strictly anoxic. Anoxia on continental margins develops
through increased upwelling resulting in higher productivity and expansion of the
Oxygen Minimum Zone [OMZ; e.g., Takashima et al., 2006]. However, the presence of radiolarians during OAE events suggests the absence of a strong OMZ, hence, ocean acidification may have had a stronger factor on the abundance of planktic forams..
Ocean acidification typically has a stronger effect on surface dwellers relative to deeper
36 ones causing a stronger biocalcification failure in the mixed layer relative to deeper
sections of the water column [Erba et al., 2015] . The dominance of radiolarians over
planktic forams in OAEs may suggest the presence of upper water column acidification
as silica based protists are more resistant to ocean acidification.
Using the P/B and P/R ratios and the foraminifera number, eight additional acidification events may exist in the Albian section of ODP Site 763. The variable occurrence frequency of acidification events does not reflect systematic cyclicity as observed in plots of acidification events versus age (Figure 23). The poor preservation
of upper Aptian sample 37X-6, 530.25 mbsf, is consistent with acidification due to
temporal proximity with the correlative Kilian level.
37
Benthics Planktics Radiolarians Planktics
Figure 22. All OAEs (except for Leenhardt), coincide with increased dominance of radiolarians and benthics over planktics. Ocean acidification may have caused severe biocalcification failure in the upper water column. 38
Figure 23. All OAEs may be associated with ocean acidification due to low foraminifera counts and P/B and P/R ratios. 13 acidification events, including 5 that correlate with OAEs are proposed for the study interval. All suggested acidification events do not occur on systematic cyclicity.
39 3.9 Paleoenvironment
3.9.1 Micropaleontology
A paleobathymetric model of Cretaceous benthic foraminifera from the Australian
continental margin [Scheiberova, 1976] suggests the dominance of outer neritic to upper
bathyal taxa in the studied interval of ODP Site 763 (Figure 24). The presence or
absence of benthic genera from different bathymetric ranges can facilitate
paleoenvironmental interpretations. For example, the absence of Gaudryina, an
agglutinated benthic foraminifera exclusive to outer neritic settings may suggest the
falling or rising sea levels depending on the changes in benthic assemblages. In addition,
the exclusive presence of Gyroidinoides, a middle to bathyal calcareous benthic
foraminifera, with inoceramid prisms in sample 33X-3, 487.7 mbsf, 110.04 Ma suggests
a middle neritic environment for inoceramids due to the absence of all outer neritic and
bathyal benthics. In addition, inoceramid prisms could also be outer neritic as the
abundance of Gaudryina and inoceramid prisms decreased significantly between (467 –
468 mbsf, 105.37 – 105.75 Ma), despite an increase in benthic abundances overall, suggesting a co-habitation between Gaudryina and inoceramids (Figure 25). Therefore, the dominance or absence of inoceramid prisms may indicate changes in sea level, due to their inferred presence in neritic environments.
Infaunal benthic foraminifera can indicate low or high carbon fluxes based on
observed adaptations of modern foraminifera and inferred preferences from similar
geological settings in the same time interval [e.g., Holbourn, et al., 2001]. The percentage
of Dorothia oxycona and Gaudryina (infaunal benthic foraminifera adapted to low
carbon fluxes) increases upwards which is consistent with gradually decreasing values
of δ13C and rising sea levels as bottom waters had lower organic carbon fluxes (Figure
40 26).
Slower sedimentation rates and higher sea level are associated with higher benthic
foraminifera abundances in the mid-late Albian (486 – 456 mbsf, 110 -113.12 Ma ) which coincides with the “inoceramid acme”. In addition, the abundance of inoceramid prisms is associated with a decline in radiolarians and planktic forams (Figure 27).
Since inoceramid prisms indicate neritic environments, their abundance may reflect falling sea levels associated with a decline in planktic and radiolarian abundances and vice-a-versa.
41 Figure 24. A paleobathymetic model of benthic genera in the Australian margin suggests that most benthic foraminifera formed in outer neritic to upper bathyal marine conditions. Adapted from [Scheiberova et al., 1976] 42 “Inoceramid acme”
Inoceramids Gaudryina decrease decreases
Gyroidinoides Inoceramids
increases increase
Figure 25. Inoceramids indicate a middle to outer neritic bathymetry. A simultaneous decrease in Gaudryina and inoceramid prisms despite increasing benthic counts suggests rising sea levels and a decreasing influence of outer neritic components on the study interval. Inoceramids could also be middle neritic due to the exclusive presence of Gyroidinoides with inoceramids suggesting decreasing sea levels as all bathyal and outer neritic benthics disappear.
43
Figure 26. A gradual increase in infaunal benthics (Gaudryina and Dorothia) suggests lower oxygenation and increasing organic matter flux to the sea floor. This is consistent with a gradual decrease in benthic δ13C and rising mean sea levels.
44 Inoceramid Prism Abundances as Sea Level Indicators
Decreasing abundance TST
Decreasing abundance Decreasing abundance HST
Decreasing abundance TST
Decreasing abundance Decreasing abundance HST
Decreasing abundance TST
Figure 27. The abundance of inoceramid prisms is associated with a decline in planktic and radiolarian abundances suggesting that inoceramid prisms may reflect changes in sea levels. Inoceramid prisms suggesting the development of neritic environments or downslope transport from neritic environments during a sea level fall.
45 3.9.2 Stable Isotope Record
• 563 – 530 mbsf (Upper Aptian)
The isotopic composition of late Aptian bulk FF samples is variable in both oxygen and carbon isotopes possibly due to the abundance of smectite muds, which can cause significant diagenetic variations in isotope compositions [Figure 15; Exon et al., 1992;
Price et al., 2012] . However, SST estimates from bulk FF oxygen isotopes did not exceed
17°C while benthic temperature estimates did not exceed 14°C in Cores 41X-39X (563 –
543 mbsf, 114 – 113.4 Ma). Such low SST values are associated with the late Aptian
cooling (116-114 Ma) and long-term low stand (118-113 Ma) [McAnena et al., 2013;
Maurer et al., 2012].
Core 38X (533 – 532 mbsf, 113.09 – 113.06 Ma) shows positive δ13C excursion in
benthic and bulk fine fraction values. However a positive δ18O excursion occurs in the
benthic and planktic record but not in bulk FF. A late Aptian transgression may coincide
with this positive δ13C excursion as suggested by Price (2012) in late Aptian δ13C record
of belemnites in the Carnarvon Basin, Australia.
• 530 – 482 mbsf (Lower to Middle Albian)
Samples 37X-6 and 37X-5 (530.25 and 528.75 mbsf) are the uppermost Aptian and
lowermost Albian, respectively, as discussed in the Chronostratigraphy section. Oxygen
isotopes of both samples show a cooling signal across the boundary and a brief recovery
of ocean stratification associated with higher abundances of benthics, planktics,
radiolarians, and inoceramid prisms at sample 37X-5. The Haq (2014) eustatic sea level
curve suggests a falling sea level across the Aptian/Albian boundary, which may be
consistent with a cooler temperatures seen in ODP Hole 763B. The high abundance and
diversity of biocomponents in samples 37X-5 and 37X-4 may be part of a prograding
46 sequence during a sea level fall. On the other hand, increased biotic diversity and
abundance may be produced from a high-order sea level rise in the earliest Albian
within a generally regressive early Albian sequence (sea level discussion).
Reworking of late Aptian planktic forams has been observed by Huber and Leckie
(2011), as Aptian species co-occur with early Albian forms as high as Sample 36X-CC
(522.67 mbsf, 112.5 Ma). Lower abundance, severe dissolution and mixed δ18O signals are also observed in Osangularia through this interval of reworking (530.25 – 522.67 mbsf, 112.95 – 112.5 Ma). Reworking can be a feature of an early transgressive systems tract, while the abundance of radiolarians in the overlying interval (519 – 505 mbsf,
112.3 – 111.1 Ma) is more consistent with a late transgressive systems tract.
Alternatively, downslope transport of benthic foraminifera is consistent with a significant sea level fall on a continental slope. Therefore, falling sea level may have reworked Aptian and early Albian benthic and planktic forams from neritic to upper bathyal settings.
Oxygen isotope gradients increase and stabilize between (505 – 492 mbsf, 111.1 –
110.4) which, combined with the dominance of radiolarians, suggests a late transgressive and highstand sequence with normal marine stratification. This is followed by a falling sea level between (492-482 mbsf, 110.2 -109.1 Ma) as suggested from higher abundances of inoceramid prisms and Gyroidinoides deposited in a middle to outer neritic environment. Temperature estimates suggest that the benthic realm warmed by ~4°C in an interval (492 – 482 mbsf, 100.2 – 109.1 Ma) where inoceramid prisms decrease.
• 482 – 448 mbsf (Middle to Upper Albian)
Oxygen and Carbon isotope gradients decline significantly in three pulses at (479, 467 and 451 mbsf, 108.37, 105.4 and 102.6 Ma). An important observation is the increased
47 abundance of planktic, benthic, and radiolarians coincide with increased water column
mixing as indicated by the lower temperature gradients indicated by lower oxygen
isotope gradients.. Negative carbon excursions in both benthic and bulk FF suggest an
upwelling signal and increased productivity, but not an OAE. Typical upwelling events
are associated with negative carbon excursions in benthic and planktic foraminifera due
to the influence of 13C depleted, nutrient-rich waters [Pak and Kennett, 2002].
The coincidence of upwelling with increasing abundances of radiolarians and planktic
forams with rising sea levels interpreted based on a decline in inoceramid prisms
suggests a causal effect. Three pulses of warmer bottom waters suggest a sea level
controlled invasion of temperate, nutrient-rich water masses in the middle to early late
Albian affecting Site 763. Perhaps these warming pulses are due to the production of
warm, saline intermediate waters along the subtropical northern margin of Australia. In
addition, carbon and oxygen isotope gradients increase and cooler temperatures
alternate with the warming events, and coincide with increased inoceramid prism
abundances and falling sea level.
A strong correlation between oxygen isotopes and 3rd order eustatic sea level cycles
has been observed in a long ranging Cretaceous record in the Contessa section in Italy
[Haq, 2014]. The time interval between the three oxygen isotope excursions at (108.37,
105.4 and 102.6 are 2.98 and 2.74 Ma consecutively which is typical of 3rd order eustatic
sea level cycles which can range from 0.5 to 3 myr [Haq, 2014]. Therefore, 3rd order eustatic cycles resulted in invasion of warmer bottom waters, decreased stratification, increased upwelling and higher planktic and radiolarians populations during transgressions. Regressions resulted in cooling, increased inoceramids on the shelf, increased stratification and decreased abundance of planktic and radiolarian populations.
48 • 448 – 410 mbsf (Uppermost Albian)
Carbon and oxygen isotope gradients achieve the largest values in the uppermost
Albian, indicating the strongest water column stratification in the studied section. Pyrite nodules become dominant in the interval (447 – 425 mbsf, 102.2 – 100.5 Ma)
suggesting the development of euxinic conditions on the seafloor and in the water
column as planktic, benthic, and radiolarian populations are almost absent. Despite the
high abundance of pyrite in several samples, euxinic conditions may have only formed
in two horizons, one of which coincides with OAE1d placed at 427.73 mbsf, 100.74 Ma
and the other at 440 mbsf, 101.7 Ma. The presence of euxinia in an intensely stratified
water column suggests that higher sea levels in the late Albian were associated with
bottom water anoxia possibly caused by increased rainfall and decreased salinity in
surface waters resulting in a weakened ocean circulation [Wilson and Norris, 2001;
Bornemann et al., 2005].
The Albian/Cenomanian boundary placed at (425 mbsf, 100.5 Ma) coincides with an
abrupt decline in planktic abundances and pyrite nodules, while radiolarian abundances increase possibly due to increased acidification. The Albian/Cenomanian boundary shows no decline in water column stratification resulting in a similar susceptibility to bottom water anoxia and/or euxinia. However, the lack of pyrite and increased frequency of acidification events may have reduced the presence of organic matter and subsequent H2S resulting in decreased abundances of pyrite. A decline in organic
matter, implied from a decline in pyrite, suggests that limited nutrients may have
caused a decline in planktic abundances and an increase in radiolarians, in addition, to
acidification possibly caused by the formation of the central Kerguelen Plateau starting
in 102 Ma (www.ga.gov.au, retrieved November 2015).
A Tethyan influence is inferred in the interval between (456 – 413 mbsf, 103 – 98.5
49 Ma) based on the incursion of keeled planktic foraminiferal taxa, including Planomalina buxtorfi and species of Rotalipora and Praeglobotruncana, which are typical of low latitudes [Caron, 1985, 1989; Premoli Silva and Sliter, 1999] . In addition, the dominance of heavily calcified, keeled planktics indicate the development of a deep thermocline indicating further stratification in the upper water column [Wilson and Norris, 2001;
Leckie et al., 2002].
50
Figure 28. A composite plot of isotope data and mineral and biocomponents with sequence stratigraphic interpretations. The abundance of neritic inoceramid prisms associated with a cooler, more stratified water column indicates falling sea levels and increased delivery from neritic environments into bathyal settings. Increased abundances of planktic and radiolarian populations associated with decreased gradients, increased upwelling and negative carbon excursions are probably caused by rising sea levels which lead to the invasion of warmer bottom waters which increase upwelling.
51
Figure 29. A composite plot of isotope, mineral and biocomponent data with acidification events. Two horizons with euxinic conditions, one of which is OAE1d, replace two acidification events as the abundance of pyrite and lack of planktic, benthic and radiolarian populations suggests severe anoxia. The presence of deep thermocline keeled planktics and increased temperature stratification in δ18O values from the late Albian (112.6 Ma) to early Cenomanian suggest slower ocean circulation assisting the development of euxinic conditions.
52 3.10 Comparison of Isotope Records of Bulk Carbonate to Bulk Fine Fraction Carbonates and Benthic Foraminifera
A bulk carbonate oxygen isotope record from the Aptian-Cenomanian interval
from Site 763B published by Clarke et al., 1999, is compared to the bulk fine fraction and benthic foraminifera isotope data generated in this study. Carbon isotope data of bulk carbonates was not previously published, but was provided by Dr. Leon
Clarke (MMU) for comparison with data produced from this study. Bulk fine fraction data can be more representative of the mixed layer and photic zone compared to bulk carbonates because of a presumed greater concentration of autotrophic calcareous nannofossils and fewer heterotrophic planktic and benthic foraminifera.
Therefore, the bulk fine fraction is expected to be more enriched in δ13C and more
depleted in δ18O [Reghellin et al., 2015]. Indeed, our bulk fine fraction records were
slightly more enriched in δ13C, but significantly more depleted in δ18O compared to
Clarke’s bulk carbonate data.
3.10.1 Carbon Isotopes
Clarke’s bulk carbonate δ13C values are remarkably similar to our bulk fine
fraction values, while δ18O values show a larger offset that increases in the upper
Albian and Cenomanian. However, despite the similarity of the two carbon isotope time-series, which both record Excursions 2, 3 and 4, there are two excursions (5 and 6) that only show up in the bulk carbonate data (Figure 30). Perhaps Excursions
5 and 6 were short-lived and not represented in the fine fraction analyses of this study. The time difference between Excursions 5 and 6 suggests third-order cyclicity
53 similar to the earlier Excursions 2, 3 and 4 in the middle to upper Albian. An
additional difference in the two C-isotope time-series is that Clarke’s bulk carbonate data do not record a prominent negative excursion at the Aptian/Albian boundary
(530 mbsf, Excursion 1) probably due to his lower sampling resolution near the
boundary interval, which contains a 1-m sampling gap between the lowest Albian sample at 529.24 mbsf and the boundary at 530.28 mbsf.
3.10.2 Oxygen Isotopes
Clarke’s bulk carbonate δ18O values are more enriched than our bulk fine fraction
data, which is consistent with the likely greater presence of benthic foraminifera
that formed in deeper, cooler water settings resulting in more enriched values. The
sieving process of the bulk fine fraction samples removes a greater proportion of
benthic and planktic foraminifera thereby concentrating the calcareous nannofossils
and resulting in slightly more depleted δ18O values compared to unsieved bulk
carbonates.
The Aptian/Albian boundary (530 mbsf, Excursion 1) shows a prominent and
simultaneous positive excursion in the δ18O record of benthic and bulk fine fraction
carbonate values suggesting significant bottom water cooling across the boundary
and into the early Albian. This is consistent with cooling temperature trends from an
oxygen-isotope study of belemnite guards from the lower Albian Gearle Siltstone of
the Carnarvon Basin of western Australia, paleolatitude 40-45°S [Williamson, 2006].
However, oxygen isotope values of the Aptian/Albian boundary Niveau Kilian in the
Vocontian basin exhibit a warming trend [Petrizzo et al., 2012; Huber et al., 2011]. On
54 the other hand, the dominance of terrestrial organic matter within Kilian in the
Vocontian Basin of southeast France suggests a falling sea level facilitating the
delivery of terrestrial organic matter into a marine setting [Herrle, 2003; Herrle,
2004; Okano, 2008]. In addition, correlative Kilian level angular quartz-rich turbidites were found within hemipelagic sediments in the deep Newfoundland
Basin suggesting falling sea levels [Trabucho et al., 2011].
Clarke’s bulk δ18O shows three excursions that coincide with benthic δ18O
excursions (2, 3 and 4). However, bulk fine fraction δ18O excursions are variable;
Excursion 3 is positive while excursions 2 and 4 are negative. In addition, excursion
2 has two negative prongs; one coincides with the benthic excursion and the other
coincides with bulk fine fraction excursion. The presence of two excursions in bulk
carbonate δ18O isotopes indicates the validity of both excursions and confirms the
presence of a vertical offset between benthic and bulk fine fraction excursions.
Therefore, bulk carbonate δ18O values could reflect excursions in the benthic or planktic realms or both. In addition, bulk carbonate δ18O values show a smaller
excursion than benthic and bulk fine fraction values suggesting that bulk δ18O values may reflect an average of the benthic and planktic realms.
The increased offset between bulk and bulk fine fraction δ18O values in the
interval between 447 to 410 mbsf (uppermost Albian and lower Cenomanian)
coincides with a significant increase in the abundance of deeper dwelling keeled
planktic taxa, which are more enriched than typical mixed layer nannofossils
[Reghellin et al., 2015] (Figure 31). Therefore, bulk carbonates generally represent
mixed layer nannofossils, however, benthic and deep dwelling planktic foraminifera
55 may have a significant impact on the isotopic composition of bulk carbonates at a time when tropical-subtropical thermocline taxa invaded the high southern latitudes due to global warming and higher sea level during latest Albian time [e.g.,
Leckie et al., 2002].
56 Warmer Cooler Excursion 6
Excursion 5
Excursion 4 Excursion 4
Excursion 3 Excursion 3
Excursion 2 Excursion 2
Excursion 1 Excursion 1
Figure 30. A composite record of data produced in this study compared with Clarke’s bulk carbonate isotope values. The δ13C values correlate well except for Excursion 1 at the Aptian/Albian boundary showing an attenuated excursion in bulk carbonates, while Excursions 5 and 6 are only prominent in bulk carbonates. Bulk carbonate oxygen isotope values are more enriched than bulk fine fraction values due to the presence of benthic foraminifera from a colder and deeper setting. A positive δ18O excursion at the Aptian/Albian boundary (Excursion 1) supports the presence of cooler benthic conditions while bulk fine fraction data suggest generally cooler conditions during the latest Aptian compared with an overall warming during the Albian.. 57
Figure 31. Clarke’s bulk isotope data are plotted with bulk fine fraction data from this study. Bulk fine fraction values are generally more enriched in δ13C values and more depleted δ18O values than bulk carbonates. The gradient between bulk and bulk fine fraction carbonates is mostly due to the presence of benthics in Clarke’s bulk carbonate records. However, the gradient is larger in δ18O values and increases significantly between 448 and
58 3.11 Paleoenvironment across the Aptian/Albian OAE1b.
Negative carbon isotope excursions were observed in bulk carbonates and
organic matter from the Niveau Kilian black shale in the Vocontian Basin, and from
correlative black shales on the Mazagan Plateau and in the deep Newfoundland
Basin [Herrle et al., 2004; Alexandre, 2011] (Figure 32). Negative excursions may be
caused by a rapid reduction in productivity or an abrupt global change in the carbon
cycle due to the input of isotopically light carbon from terrestrial organic matter,
volcanogenic carbon dioxide, and/or the liberation of methane from methane
hydrates. Volcanic eruptions can produce massive amounts of CO2, which typically
has a carbon isotopic composition between -2 to -6‰ [Faure & Mensing, 2005].
Methane, on the other hand, is much more depleted than carbon dioxide with carbon isotope values reaching -60‰ for biogenic sources and -40‰ for thermogenic sources [Faure and Mensing, 2005]. The implication is that it would take much less methane injected into the ocean-atmosphere system to cause a negative C-isotope excursion than if caused by volcanism [e.g., Dickens et al., 1995;
DeConto et al., 2012].
The depleted isotopic composition of carbon dioxide is incorporated in calcite through the interaction of atmospheric CO2 with seawater molecules forming depleted bicarbonate ions, which in turn combines with calcium ions forming depleted calcium carbonate:
1. CO2(aq) + H2O H+ + HCO-3
2. HCO-3 + Ca2+ CaCO3 + CO2(aq) + H2O
59 Methane hydrates form along productive continental margins due to the
decay of organic matter by methanogenic bacteria under relatively high pressure
and low temperatures. Methane may be liberated from its solid hydrate form to a
gaseous form if methane hydrates are destabilized by warmer deep or intermediate
waters, which may have formed during the warmer Cretaceous climates [e.g.,
Thierstein and Berger, 1978; Brass et al., 1982; Arthur et al., 1987; Schmidt and
Mysak, 1996]. Methane may react with dissolved oxygen in the water column forming heavily depleted CO2.
CH4 + 2O2 CO2(aq) + 2H2O
The presence of negative C-isotope excursions in black shale equivalent to
the Aptian/Albian boundary Niveau Kilian suggests that while primary productivity
may have increased during the deposition of black shales, the effect of isotopically
light carbon supersedes carbon isotope enrichment caused by elevated primary
productivity and burial in the sedimentary record. In addition, the Kilian interval
may have been dominated by eroded isotopically depleted terrestrial organic matter
type III, which may produce negative isotope excursions. However, the presence of a
negative carbon isotope excursion across the Aptian/Albian boundary in ODP Hole
763B suggests that in the absence of organic matter-rich black shale, the observed
negative excursion is more likely due to changes in the global budget of carbon
isotopes.
60
Niveau Paquier
equivalent horizons
Niveau Kilian equivalent horizons
Figure 32. The Aptian/Albian boundary Niveau Kilian black shale in the Vocontian Basin (lower middle plot) and equivalent horizons in the Mazagan Plateau (lower right) and the Newfoundland Basin (left) show negative excursions due to the input of isotopically light carbon from a combination of possible sources, including volcanogenic CO2 associated with Kerguelen Plateau volcanism, liberation of methane hydrates due to rising bottom water temperatures, and/or erosion of terrestrial organic due to falling sea level. (Figure from Alexandre et al., 2011).
61 3.12 Late Aptian Cooling
Cooling during the late Aptian has been described by many authors [e.g.,
Lini and Weissert, 1991; Mutterlose et al., 2009; McAnena et al., 2013]. The most
recent complete record of late Aptian cooling was published by Bottini et al., 2015
showing two phases of cooling, the first in the early late Aptian following OAE1a,
and the second in the latest Aptian preceeding the Aptian/Albian boundary.
Temperature estimates from oxygen isotopes of bulk fine fraction samples from
ODP Hole 763B show temperature ranges that vary between 11° – 16°C with
warmer temperatures closer to the Aptian/Albian boundary. Despite the likely
diagenetic alteration of oxygen isotope values and significant core gaps, the range of
temperatures is much cooler than typical Cretaceous sea surface temperatures [e.g.,
Norris and Wilson, 1998; Huber et al., 2002; Leckie et al., 2002; Friedrich et al., 2012].
3.13 Pyrite and Siderite as Expressions of Anoxia
The abundance of siderite (FeCO3) and pyrite (FeS2) may indicate that
intensified anoxia developed sporadically in the late Aptian and latest Albian along
the margin of northwest Australia. Siderite is abundant in the upper Aptian during
prolonged low-stand sea level, while pyrite formed in the uppermost Albian when
sea levels were significantly higher. Both minerals require the presence of organic
matter and Fe(III), which may be have been supplied by hydrothermal fluids in the
late Aptian and late Albian during the formation of the southern Kerguelen Plateau
(119 – 110 Ma) and central Kerguelen Plateau starting around 102 Ma [Leckie et al.,
2002; Erba et al., 2015].
62 Hydrothermal Fe(II) can be removed from solution if sulfides (HS- and H2S)
are abundant in the water column, due to increased degradation of organic matter,
resulting in the production of insoluble iron sulfide; pyrite (FeS2) [Kendall, 2012].
Fe2+ + HS FeS + H+
FeS + H2S FeS2 + H2
However, when hydrothermal Fe(II) concentrations exceed sulfide, iron may
oxidize to Fe(III) in surface waters, followed by hydrolysis forming Fe(OH)3 and in turn (FeCO3) in the presence of organic matter in reducing conditions [Kendall et al.,
2012; Kohler et al, 2013] (Figure 35)
2+ 4Fe(OH)3 + CH2O FeCO3 + 3Fe + 6OH + 4H2O
63
Figure 35. Hydrothermal activity associated with the Southern Kerguelen Plateau could theoretically have supplied large amounts of Fe(II) in the late Aptian, which oxidized in shallower environments forming Fe(III), followed by hydrolysis forming Fe(OH)3 and sinking to the ocean floor. Under, anoxic conditions, Fe(OH)3 can react with organic matter forming siderite. (Figures from Kendall et al., 2012 and Kohler et al., 2013).
64 CHAPTER 4
CONCLUSIONS
The late Aptian cooling may have affected the Site 763B as suggested by sea
surface temperatures (~11° - 16°) calculated from oxygen isotope values of bulk fine
fraction carbonates. The variability of oxygen isotope values suggests a diagenetic
influence, however, the temperature range is significantly lower than typical Cretaceous
sea surface temperatures. A 5-myr glacioeustatic(?) lowstand in the late Aptian may
have assisted the development of siderite under anoxic conditions on the continental
slope as hydrothermal fluids from the southern Kerguelen hotspot supplied iron that
reacted with organic matter forming iron carbonate (siderite).
The Aptian/Albian boundary and the correlative Niveau Kilian black shale can
be identified by a prominent negative C-isotope excursion, the disappearance of pore
mound Aptian planktic foraminiferal ultrastructure, and the lowest occurrence of
Microhedbergella renilaevis and Mi. rischi. The absence of organic matter-rich black
shale associated with the correlative Kilian suggests that a negative carbon isotope
excursion can be observed in carbonates due to the introduction of isotopically light
volcanogenic CO2, terrestrial organic matter, and/or methane. Carbon isotope
excursions were not observed in other OAE1b (lower Albian) black shale equivalents;
Niveau Paquier and Niveau Leenhardt. However, the correlative Paquier and Leenhardt
levels coincided with severe ocean acidification based on the severe reduction in the
number of foraminifera, in general, and planktic foraminfera, in particular, and the dominance of radiolarians in the upper water column. Ocean acidification also coincides with correlative upper Albian OAE1c and 1d levels, in addition to several horizons, not typically associated with Albian oceanic anoxic. Therefore, ocean acidification may have occurred more frequently in the mid-Cretaceous than oceanic anoxic events. Ocean
65 acidification probably occurred due to the enhanced production of volcanogenic carbon dioxide from the Kerguelen Plateau and other sources.
The Albian interval in ODP Hole 763B contains an assemblages of benthic foraminifera that suggest a wide paleobathymetric range from middle neritic (100 m) to upper bathyal (600 m) water depths. However, cycles of abundant inoceramid prisms, which may have proliferated in strictly neritic paleobathymetric zones, are used to infer third-order sea level cycles. In addition, inoceramid prisms were abundant during cycles of cooler and more stratified water columns, which may have been associated with falling sea levels leading to the redeposition of neritic inoceramids on the continental slope. In addition, inoceramid prisms decrease abruptly during rising sea levels leading to the invasion of warmer subtropical intermediate waters causing enhanced upwelling. Upwelling is inferred from reduced oxygen isotope gradients interpreted as weaker temperature stratification of the water column. Upwelling is also evident in lower carbon isotope gradients between benthic foraminifera and bulk fine fraction values as upwelling supplies isotopically depleted nutrients to surface waters leading to negative excursions in both benthic foraminifera and bulk fine fraction.
The lates Albian – early Cenomanian interval exhibits the appearance of and alternating dominance between thermocline dwelling keeled planktic foraminifera and pyrite nodules. Keeled planktic foraminfera indicate the influence of a war, Tethyan water mass and the development of a deep thermocline while pyrite may have been deposited under euxinic conditions during intensified stratification. Both interpretations can be consistent with globally rising sea levels in the middle
Cretaceous.
Comparison of bulk and bulk fine fraction carbon isotope values indicates remarkably similar values while oxygen isotopes are more enriched than bulk
66 carbonates values. The depleted values of bulk carbonates suggest the influence of depleted benthic foraminifera, which were eliminiated from the fine fraction during the sieving process. The offset between bulk and bulk fine fraction oxygen isotope values increases in the late Albian to early Cenomanian, which is consistent with the presence of thermocline dwelling keeled planktic foraminifera in bulk carbonates.
Siderite and pyrite represent expressions of anoxia during intensified stratification associated with the late Aptian lowstand and the late Albian transgression.
The formation of both minerals requires anoxia and the presence the presence of hydrothermal iron, which may have been supplied by the South Kerguelen (late Aptian- early Albian) and Central Kerguelen (late Albian) hotspot volcanism, respectively.
67 APPENDIX
AGE MODEL RESULTS
Interval Interval Depth Age Core # Section # top Core # Section # top Depth Age 408.72 97.69 25 1 22 28 3 23 440.23 101.72 410.235 97.96 25 2 21.5 28 4 24 441.76 101.85 411.73 98.22 25 3 23 28 6 23 444.75 102.09 413.25 98.49 25 4 23 28 7 25 446.25 102.21 414.73 98.76 25 5 23 28 CC 21 446.62 102.24 416.25 99.02 25 6 23 29 1 23 446.73 102.24 417.73 99.29 25 7 23 29 2 24 448.27 102.37 418.05 99.34 25 CC 23 29 3 23 449.73 102.48 418.23 99.38 26 1 23 29 4 23.5 451.255 102.61 419.75 99.65 26 2 23 29 5 23 452.73 102.72 421.23 99.91 26 3 23 29 6 19.5 454.22 102.84 422.74 100.18 26 4 22 29 7 23 455.73 102.96 424.23 100.44 26 5 23 29 CC 23 456.17 103.00 424.5 100.47 26 CC 3 30 1 23 456.23 103.00 26 CC 23 424.68 100.48 30 2 25 457.77 103.13 27 1 23 427.73 100.73 30 3 23 459.23 103.47 27 2 23 429.25 100.85 30 4 23 460.75 103.83 27 3 23 430.73 100.97 30 5 24 462.24 104.19 27 4 22.5 432.245 101.09 30 6 26 463.78 104.56 27 5 23 433.73 101.20 30 7 24 465.24 104.91 27 6 24 435.265 101.33 30 CC 24 465.61 104.99 27 7 23 436.73 101.44 31 1 24 465.74 105.03 27 CC 23 437.04 101.47 31 2 23 467.25 105.39 28 1 23 437.23 101.48 31 3 23 468.73 105.74 28 2 25.5 438.775 101.61 31 4 25 470.275 106.11
68 Interval Interval Core # Section # top Depth Age Core # Section # top Depth Age 31 5 23 471.73 106.46 34 CC 23 503.67 111.17 31 6 23 473.25 106.82 35 1 23 503.73 111.18 31 7 23 474.73 107.17 35 2 26.5 505.285 111.29 31 CC 23 475.09 107.26 35 3 23 506.73 111.39 32 2 23 476.75 107.65 35 4 25 508.27 111.50 32 3 23 478.23 108.01 35 5 21 509.71 111.60 32 4 22 479.74 108.37 35 CC 23 512.73 111.81 32 5 23 481.23 108.72 36 1 25 513.25 111.85 32 6 26.5 482.79 109.10 36 2 22 514.74 111.96 32 7 22 484.22 109.44 36 3 23 516.23 112.06 32 CC 23 484.64 109.54 36 4 25 517.77 112.17 33 1 25 484.74 109.56 36 5 23 519.23 112.27 33 2 31 486.33 109.94 36 6 23 520.75 112.38 33 3 23 487.73 110.04 36 7 23 522.23 112.49 33 4 25.5 489.275 110.15 36 CC 23 522.67 112.52 522.73 112.52 33 5 24 490.74 110.25 37 1 23 37 2 26 524.28 112.63 33 6 23.5 492.255 110.36 37 3 24 525.74 112.74 33 7 23 493.73 110.47 37 4 26.5 527.285 112.85 33 CC 23 494.08 110.49 37 5 23 528.73 112.95 34 1 22 494.22 110.50 37 6 26 530.28 113.00 34 2 21 495.73 110.61 37 CC 23 530.73 113.01 34 3 23 497.23 110.71 38 1 25 532.25 113.06 34 4 22 498.74 110.82 38 1 29 532.32 113.06 34 5 27 500.27 110.93 38 1 78 532.8 113.07 34 6 28 501.8 111.04 38 1 105 533.07 113.08 34 7 23 503.23 111.14 38 CC 19 533.45 113.09
69 Interval Core # Section # top Depth Age 41 2 73 562.75 113.97 38 CC 23 533.47 113.09 41 2 85 562.87 113.98 39 1 12 541.64 113.34 41 2 100 563.02 113.98 39 1 20 541.7 113.34 41 CC 5 563.3 113.99 39 1 56 542.08 113.35 41 CC 23 563.46 113.99 39 1 73 542.25 113.36 39 1 82.5 542.345 113.36 39 1 136 542.88 113.38 39 2 9 543.11 113.38 39 CC 22 543.55 113.40 40 1 20 551.22 113.63 40 1 23 551.23 113.63 40 1 89 551.91 113.65 40 1 114 552.16 113.65 40 1 140 552.42 113.66 40 2 16 552.68 113.67 40 2 100 553.52 113.70 40 CC 17 553.94 113.71 40 CC 25 553.99 113.71 41 1 10 560.62 113.91 41 1 25 560.75 113.91 41 1 30 560.82 113.92 41 1 50 561.02 113.92 41 1 65 561.17 113.93 41 1 100 561.52 113.94 41 1 120 561.72 113.94 41 1 140 561.92 113.95 41 2 15 562.17 113.96
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